The present invention relates to the field of high-strength low-alloy steel, and more particularly to compositions and methods for making high-strength low-alloy steel using titanium as the only, or as a principal, microalloy element for strengthening.
High-strength low-alloy (HSLA) steels conventionally use the alloying elements of vanadium, niobium, or combinations thereof for precipitation strengthening and grain refinement. Titanium is also used in combination with these elements. Relatively small amounts of the alloying elements, generally up to 0.10% by weight, are used to attain a yield strength of at least 275 MPa (40 ksi) in order for the steel to be considered high-strength. Of these alloying elements, titanium is the least expensive.
As known, titanium added to steel serves to limit austenitic grain growth in fully killed steels. Titanium induces precipitation of several compounds that form on cooling of the steel, including titanium nitride, (TiN), titanium carbide (TiC), and titanium carbonitride (Ti(C, N)). The first to form is TiN, which has three effects. The first effect is that precipitation of TiN eliminates free nitrogen from the steel. Free nitrogen in the steel is known to reduce toughness. Second, fine dispersion of TiN in the steel matrix limits grain growth, leading to grain size refinement during reheating. Third, TiN increases impact toughness at heat affected zones that are created through operations such as welding. Precipitation of TiN in liquid steel needs to be minimized, because these precipitates can be relatively coarse and have a size of up to 1 xcexcm or more. Coarse TiN precipitates can have negative impacts on the steel because they are sharp-angled and relatively few in number, limiting the hardening and refining of the microstructure and degrading toughness and ductility. For the purpose of TiN formation, it is conventionally thought that titanium content should not exceed 0.03% by weight in order to minimize TiN precipitation in liquid steel, along with its detrimental effects.
Formation of titanium carbides and carbonitrides requires additional titanium in the steel, and because of the limitation placed on the titanium content, are generally not substantially present. Vanadium and niobium carbides, nitrides, and carbonitrides are the primary precipitate strengthening agents in microalloyed steel.
Also as known, thin slab casting is an improvement over conventional thick slab casting, both of which may be done as continuous casting processes. Thin slabs are cast in thicknesses generally ranging from 25 to 100 mm (1 to 4 inches), while thick slabs are generally from 200 to 300 mm (8 to 12 inches). Both thick and thin slab continuous casting generally involve the steps of smelting the steel in either a Basic Oxygen Furnace or an Electric Arc Furnace, tapping the furnace into a ladle, continuing to heat the steel in the ladle in a Ladle Metallurgy Furnace, where alloys are added to create the desired chemical composition, and transferring the steel from the ladle to a tundish from which the steel flows through a water-cooled mold. The steel begins to solidify by forming a shell as it passes through the mold. Rolls downstream of the mold work with gravity to control and guide the steel strand through the mold. Thin slab casting eliminates an entire stage of processing, the roughing hot work, that is applied to thick slabs. In general after cooling and solidifying, both thick and thin slabs are reheated and hot-rolled, using various processes of controlled rolling. The temperature of the steel may be reduced by a combination of air cooling and quenching with sprayed water. A combination of controlled rolling and accelerated cooling may be performed that is referred to as thermomechanical controlled processing, and such processing may be used to attain desired characteristics and microstructure in the steel. The rolled steel is then coiled.
Titanium is conventionally thought to be inadequate to attain higher yield strengths in thin slab casting without being used in combination with vanadium or niobium. In general, such thin slab cast, low carbon microalloy steels have a microstructure of polygonal ferrite combined with pearlite, and sometimes combined with bainite. An additional desirable microstructure that may be achievable through controlled rolling with addition of niobium or vanadium is acicular ferrite. Acicular ferrite, when combined with polygonal ferrite, results in steel with improved strength and toughness.
Accordingly, a process is needed to make HSLA steel with titanium, a less expensive alloy for strengthening than either vanadium or niobium, without the expensive processing that is required by conventional thick slab casting. The steel produced should have a microstructure providing desired high strength and other beneficial characteristics.
According to the present invention, a composition and method of making a high-strength low-alloy hot-rolled steel sheet, strip, or plate bearing titanium as the principal or only microalloy strengthening element are provided. The steel is substantially ferritic and has a microstructure that is at least 20% acicular ferrite, and has a minimum yield strength of 345 MPa (50 ksi). The steel is continuously cast, hot-rolled carbon steel with high strength and having a chemical composition by percent weight including:
0.01xe2x89xa6Cxe2x89xa60.20;
0.5xe2x89xa6Mnxe2x89xa63.0;
0.008xe2x89xa6Nxe2x89xa60.03;
0:5Sxe2x89xa6S0.5;
0.01xe2x89xa6Tieffxe2x89xa60.12;
0.005xe2x89xa6Alxe2x89xa60.08;
0xe2x89xa6Sixe2x89xa62.0;
0Crxe2x89xa61.0;
0xe2x89xa6Moxe2x89xa61.0;
0xe2x89xa6Cuxe2x89xa63.0;
0xe2x89xa6Nixe2x89xa61.5;
0xe2x89xa6Bxe2x89xa60.1; and
0xe2x89xa6Pxe2x89xa60.5,
with the balance being iron and incidental impurities. Tieff is the effective content of titanium in the cast steel, which is the content of titanium not in the form of nitrides, sulfides, or oxides. Acicular ferrite increases with increases in Tieff, as does strength.
In further accordance with the present invention, a steel is provided that has a tensile strength that exceeds yield strength by 69 MPa (10 ksi) and more. A majority of the acicular ferrite grains have an average grain size less than approximately 4 xcexcm, as measured by x-ray diffraction and calculated by the Scherrer formula based on the {110}, {200}, and {211} Bragg peaks for Fe, and increased by a factor of ten.
Steel according to the present invention may further include niobium, vanadium, zirconium, or combinations thereof, in amounts up to 0.15% by weight of each microalloying element. Such addition can result in steel having a yield strength in excess of 621 MPa (90 ksi).
In yet further accord with the present invention, a process for manufacturing a hot-rolled carbon steel with high strength is provided that includes desulfurizing and deoxidizing a molten carbon steel, adding titanium, continuously casting the molten steel as a thin slab and having a composition as recited above, hot-rolling the thin slab to an approximate final thickness of from 1.8 mm to 13 mm (0.07-inches to 0.5-inches); and quenching the final thickness of steel. Yet further, specific temperatures and cooling rates are provided in accordance with the present invention.
The steel is further provided to have approximate temperatures by reheating of from 1100 to 1180xc2x0 C. (2000 to 2150xc2x0 F.) at the start of hot-rolling, and from 14xc2x0 C. (25xc2x0 F.) above and 22xc2x0 C. (40xc2x0 F.) below the steel""s Ar3 temperature on completion of hot-rolling. The cooling rate of the steel during hot-rolling may be approximately 60 to 230xc2x0 C./min (150 to 450xc2x0 F./min). Hot-rolling further may specifically include the reducing the thickness of the steel through five or six stands of rolls and specified interstand times between reductions. At least one interstand time is inadequate to allow recrystallization of austenite, and the temperature of the steel at one or more stands is less than the temperature at which austenite will recrystallize. Then the steel is quenched at an approximate cooling rate of from 810 to 1370xc2x0 C./min (1500-2500xc2x0 F./min) to a temperature of from 560 to 620xc2x0 C. (1050 to 1150xc2x0 F.).
The slab at the end of reheating preferably has a fine average austenite grain size of approximately up to 25 xcexcm. Addition of vanadium, niobium, or a combination thereof can result in yield strengths exceeding 621 MPa (90 ksi).